The flying characteristics of aircraft, in particular of airplanes, are disadvantageously influenced by turbulence and gusts in the air masses surrounding the aircraft. In particular, a large increase in lift, low wing loadings and high airspeeds as well as low altitudes have negative influences on the turbulence and gust behavior of aircraft. These result in a deterioration to passenger comfort and in an increase in structural loads. However, strong turbulence (“clear air turbulence” CAT) can occur even at high altitudes and can produce considerable structural loads, and can even lead to danger to the aircraft occupants.
A system for reducing gust loads and for damping structural oscillations is described in O'Connel, R. F.: “Design, Development and Implementation of an Active Control System for Load Alleviation for a Commercial Airplane”, in: AGARD Report No. 683,1979 and in rolling vehicles, G.; Ellgoth, H.; Beuck, G.: “Identification of Dynamic Response, Simulation and Design of a Highly Nonlinear Digital Load Alleviation System for a Modern Transport Aircraft”, in; 17th ICAS Congress, Stockholm, Sweden, based on the principle of signal (feedback closed loop system). However, this control system reacts only after the flying characteristics resulting from turbulence and/or gusts have already notably changed.
A control method based on the principle of application of disturbance variables in order to reduce gust loads and to improve passenger comfort is known from Bohret, H.; Krag, B.; Skudridakis, J.: “OLGA—An Open Loop Gust Alleviation System”, in: AGARD CP No. 384, Toronto, Canada, 1985. In this case, the flying characteristics are not changed, with a reaction taking place to the original disturbance itself, and compensating for it before the disturbance caused by turbulence or gusts acts on the aircraft itself.
Comparable control methods are also described in Hahn, K.-U.; König, R.: “LARS—Auslegung eines fortschrittlichen Böenabminderungssystems mit ATTAS”, (LARS—design of an advanced gust reduction system using ATTAS), in: Deutscher Luft and Raumfahrtkongress, (German Aviation Space Flight Congress), 1991 and in Hahn, K.-U.; König, R.: “ATTAS Flight Test and Simulation Results of the Advanced Gust Management System LARS”, in: AIAA Atmospheric Flight Mechanics Conference, Hilton Head Island, S.C., USA, 1992.
When using the principle of signal feedback (closed loop), the reaction of the aircraft to the gusts is measured and is fed back to the wing control surfaces in order to reduce this reaction. This does not require any complex calculation of the gust angle. However, accelerations results from flight maneuvers are also fed back via the control system and can counteract the pilot commands.
In the case of open-loop control methods, the angle of attack of a gust must be known precisely. This must be determined from sensor signals. The control surfaces of the wings and of the tailplane are adjusted as a function of the gust angles of attack in such a way that additional lift forces and pitch moments caused by the gusts are compensated for. In this case, the handling characteristics of the aircraft remain unchanged. However, the efficiency of the control system is highly dependent on the accuracy of the calculation of the gust angle of attack, and on the degree of deflection of the control surfaces.
The control method based on the principle of application of disturbance variables, in which the so-called wind incidence angle is calculated from air data and inertial data is described in König, R., Hahn, K-U.: “Load Alleviation and Rights Musing Investigations using ATTAS”, in: 17th ICAS Congress, Stockholm, Sweden, 1990. The wind incidence angle is the additional incidence angle which changes the lift and results from atmospheric turbulence and gusts. Only the aircraft longitudinal movement is taken into account, in order to avoid complex gust vector determination. The wind incidence angle αW is calculated using the following formula:
      α    W    =            α      L        -    θ    +                  H        .            V        +                  q        ·                  r          s                    V      In this case, αL is the incidence angle measured by an incidence angle sensor (for example aircraft), θ is the longitudinal attitude angle, also referred to as the pitch angle, {dot over (H)} is the instantaneous vertical velocity of the aircraft, V is the airspeed of the aircraft with respect to the air, q is the pitch rate of the aircraft and rS is the distance between the wind attack sensor and the center of gravity of the aircraft.
The stated variables are defined unambiguously in DIN 9300 “Luft- und Raumfahrt; Begriffe, Gröβen und Formelzeichen der Flugmechanik” (aviation and space flight; terminology, variables and formula symbols for flight mechanics).
The pitch angle is in this case the angle between the aircraft longitudinal axis in the aircraft-fixed coordinates system and the node axis k1 as the projection of the aircraft-fixed aircraft longitudinal axis xf onto the geodetic horizontal plane, that is to say the xg-yg-plane. The pitch rate q is the angular velocity of the aircraft about the aircraft lateral axis yf.
The described control method is not suitable for adequate turbulence and gust compensation when in turning flight as a result of the simplified consideration of only the aircraft longitudinal movement, particularly when sideslip angles also occur in this case between the lateral axis and the lateral force axis of the aircraft.